Can't go fast enough, running out of gear.

I have a 26" 24 speed (3 front, 8 rear) and it's all Shimano equipped. I ride for intense exersize like up and down city hills. I never use the front small 2 crank rings, always just the 3rd. Should I just upgrade to a 10 speed cassette? What all is needed to convert to such? The cassette, rear shifter? Does the derailleur need to be changed as well? Is there anything else, or any other way I could pick up more speed down hills? Right now I max out at ~42mph on the biggest hill in town and I can't pedal fast enough to keep up with it. I can really only pedal to ~38mph effectively. Thanks.
-Thomas

The number of speeds isn't what determines how fast the bike goes when you pedal. What you want is bigger chainrings and/or smaller cogs.

Sheldon Brown's gear calculator could be useful for you. Put in what your bike currently has and for gear units choose "MPH @ 90 RPM" (or whatever RPM your cadence is). The chart that comes up will tell you how fast you go pedaling at that cadence in each of your gears. Then go back to the form and change the chainrings size and/or the cassettes and see how that changes the results.

Generally late model 10 speed cassettes are not easily converted on to older style freewheel cogs, which I'm guessing is what you have. The spacing of the frame on newer bikes is wider to make room for the extra gears, and the gear set attaches differently.

Counting the number teeth on your largest chain ring and then the number teeth on the rear cog might provide an area to modify.

When I first got a multispeed bike, I thought I was running out of gears. After discussion with the prop of the LBS, and installation of a computer with cadence function, I decided I had a long ways to go before needing bigger chain rings. Not saying this is the OPs problem, just saying. . . .

Some people are like a Slinky ... not really good for anything, but you still can't help but smile when you shove them down the stairs.

Is it not recommended to go faster than this? On a nice flat road I could use one more gear slightly taller than my 3 and 8 combo. I'll have to count the teeth when I get home.

I got this bike used, but I think it's around a 2005-06ish. It's Jeep brand frame and handlebar, but the front fork and stem seem to be different, as they are very beefy, stiff, and light. The whole bike is incredibly light for a mountain bike. I have street tires on it as well. My intention with it is to make it a mean street bike. Lighten it up more with some carbon pieces, maybe put a slightly thinner tire on it, and possibly some aero bars for the downhill runs.

Agree ^^ Get a road bike for the long road, and get a Cyclo-computer with cadence, on the flats you should be doing 80+ rpm of the crank, this will get you the exercise you are looking for trying to keep up with your pedals going town a hill will give you a case of road rash or worse.

I have a 26" 24 speed (3 front, 8 rear) and it's all Shimano equipped. I ride for intense exersize like up and down city hills. I never use the front small 2 crank rings, always just the 3rd. Should I just upgrade to a 10 speed cassette? What all is needed to convert to such? The cassette, rear shifter? Does the derailleur need to be changed as well? Is there anything else, or any other way I could pick up more speed down hills? Right now I max out at ~42mph on the biggest hill in town and I can't pedal fast enough to keep up with it. I can really only pedal to ~38mph effectively. Thanks.
-Thomas

Well I looked at the bike and the front crankset is actually an Impel Sugino (looks like Shimano) 42/34/24 so obviously I've got a lot of room to upgrade there. I think I'll try to find a Shimano 52/42/30 and a set of 700c wheels and see where that puts me.
-Thomas

Well I looked at the bike and the front crankset is actually an Impel Sugino (looks like Shimano) 42/34/24 so obviously I've got a lot of room to upgrade there. I think I'll try to find a Shimano 52/42/30 and a set of 700c wheels and see where that puts me.
-Thomas

You can't just put 700C wheels on a 26" frame. If you want to go faster it's probably worth it to buy a road bike (and based on the speeds you want to ride at a nice set of hydraulic disc brakes may be in order too!)

A lot of MTBs have rather low overall gearing, and it's no trick for a strong rider to "spin out". Like the guys say, a roadster will generally have higher gearing. Very few humans can effectively pull 53 X 11 on a level road.

Well if someone is "yanking my chain" I sure would appreciate an honest answer. I don't know everything about cycles yet so if you tell me something Ill probably believe you.

It was an arm pad, not a bone. and Im not trying to go 105mph on my bike. I just want to use my bike effectively in a downhill situation. And like I said, even on a slight incline I run out of gear. And also, I found out I have a 42t 3rd gear in front so obviously I DO need a larger one, since most road bikes are running 52-53's.

You're right 700c's probably arent a safe choice at those speeds in the city. I think I'll stick with my 26x1.75 michelin's they have really good grip and braking traction. I'll try a new crankset and see where it puts me.